replace element in list

[edgar@openmail.cc]

Hello,

I want to share and know if there is a better way (more efficient or 
clearer) to replace something within a list (I don't know LISP).  My 
code is like this, and it works for the given example.

(defun my-list-replace (obj orig new)
     "Replaces an element in a list with something else"
     (let* (
            ;; Position of the thing we need to remove
            (pos (cl-position orig obj :test 'equal))
            ;; If pos is nil, reset to zero
            (pos (if pos pos 0))
            ;; The length of the original object
            (objlen (length obj))
            ;; The elements before the element to remove
            (head (butlast obj (- objlen pos)))
            ;; The elements after the element to remove
            (trail (nthcdr (+ 1 pos) obj)))
       ;; Join (1) the sub-list before the element to be replaced
       ;; with (2) the new element and (3) the rest of the list
       (append head (append new trail))
       ))

;; Example simple
(my-list-replace '(("a" . "b") ("c" "d")) '("c" "d") '(":)"))
;; (("a" . "b") ":)")

(my-list-replace '(("a" . "b") ("c" "d")) '("c" "d") (list '("hi")))
;; (("a" . "b") ("hi"))


;; Example long
(setq org-latex-default-packages-alist
         (my-list-replace org-latex-default-packages-alist
                    '("T1" "fontenc" t ("pdflatex"))
                    (list
                     '("" "unicode-math" t ("xelatex" "xetex"))
                     '("" "lmodern" t ("pdflatex"))
                     '("QX" "fontenc" t ("pdflatex")))))

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